The aforementioned U.S. patent describes and illustrates a Prism Light Guide for transmitting or "piping" light from a central source to a remote location or locations. As used herein the term "light guide" means a light guide as described and claimed in the aforementioned U.S. patent. More particularly, the term "light guide" as used herein means a hollow structure comprised of sections having substantially planar inner and outer surfaces which are in "octature" in that each section satisfies all of the following conditions:
1. All of the inner surfaces of each section are either perpendicular or parallel to one another; PA1 2. All of the outer surfaces of each section are either perpendicular or parallel to one another; and, PA1 3. Each of the inner surfaces is at a 45.degree. angle to each of the outer surfaces. PA1 (i) a plane which contains the vector I.sub.in and which also contains the longitudinal axis vector of the guide; and, PA1 (ii) a line perpendicular to the selected internal light guide surface through which the light is refracted. PA1 (i) a plane which contains the vector I.sub.out and which also contains the longitudinal axis vector of the guide; and, PA1 (ii) a line perpendicular to the selected internal light guide surface through which the light is refracted. PA1 (i) a plane which contains the vector I.sub.m and which also contains the longitudinal axis vector of the guide; and, PA1 (ii) a line perpendicular to the selected internal light guide surface through which the light is refracted.
As explained in the aforementioned patent, light may be constrained to travel through such a light guide without escaping through the walls of the guide if the angle by which the light rays deviate from the longitudinal axis vector of the guide does not exceed a maximum angle which depends upon the refractive index of the light guide material and which can be shown to be 27.6.degree. for light guide material such as acrylic plastic having a refractive index of 1.5.
In many applications it is desirable that light escape from the light guide at selected regions along the guide, rather than simply being directed to the end of the guide or being allowed to escape continually along the length of a given surface of the guide. It is also desirable that light escape from the guide at a selected angular orientation with respect to the guide so as to achieve the optimum coefficient of utilization with minimum glare for interior lighting applications. In many cases it is particularly desirable that light escape from the guide at an angle of 90.degree. with respect to the internal planar surface of the guide section through which the escaping light is refracted. It is also desirable that the aforementioned objectives be attained with minimal attenuation or absorption of light by whatever means may be used to facilitate the escape of light from the guide at a particular selected region or regions so that light which does not escape from the guide at a particular region continues to travel along the guide to provide illumination when its escape from the guide is facilitated by further means located at another region or regions along the guide.
One way of achieving the foregoing objectives is to locate a light reflecting element such as a mirror in one of the light guide walls or within the hollow space inside the light guide. Such elements could be oriented to reflect a portion of the light within the guide (presumed to be formed of a material having a refractive index of 1.5) at an angle (measured between a vector which characterizes the light path and the longitudinal axis vector of the guide) in excess of 27.6.degree., thereby allowing the reflected light portion to escape by refraction through the wall of the guide as explained in the aforementioned patent. However, there are several disadvantages to this approach. First, since the reflected light may escape through any of the four light guide walls an external cover must be provided around those light guide wall sections through which it is desired to prevent light escapement. The cover reflects light which escapes through the light guide walls adjacent the cover back through the light guide walls for eventual direction of the light to the uncovered light guide wall section through which it may escape so as to provide illumination along the uncovered section of the light guide surface. A filter may be provided over the uncovered light guide section through which light is allowed to escape to give the escaping light a desired angular orientation with respect to the light guide surface through which it escapes.
However, the foregoing technique has some significant shortcomings. In practice, it is rarely possible to avoid absorption of less than about 25% of the incident light by the light reflecting element and/or reflective light guide cover. Furthermore, although filtration as aforesaid facilitates some control of the angular orientation of light escaping from the light guide, the extent of such control is quite limited. As previously indicated it is often desirable that light escaping from the light guide be oriented in a relatively narrow range of angles which are approximately perpendicular to the internal (planar) surface of the light guide section through which the escaping light is refracted, which is not possible with state of the art filtration techniques and materials.
The present invention facilitates precise control of the angular orientation, relative to the light guide, of light reflected from a selected region inside a light guide and refracted by the guide to a region outside the guide. This is accomplished with the aid of light reflecting elements which are precisely located within the light guide so as to (1) enable a certain fraction of the light to pass unattenuated for processing at further region(s) along the light guide; and, (2) reflect the rest of the light toward a selected light guide wall section such that, when refracted through the wall, the light escapes from the guide at a selected angular orientation with respect to the guide.